R: Grouping by week, month, quarter

In my continued playing around with R and meetup data I wanted to have a look at when people joined the London Neo4j group based on week, month or quarter of the year to see when they were most likely to do so.

We can then plug that data frame into ggplot if we want to track membership sign up over time at different levels of granularity and create some bar charts of scatter plots depending on what we feel like!